A typical printed circuit board (PCB) is formed from multiple layers of conductive and non-conductive materials. The conductive layers (i.e., signal layers) are made of copper or a copper alloy, while the non-conductive layers (i.e., insulating layers) are made from organic polymeric materials, reinforced epoxies, and the like. Signal traces are formed on the signal layers to couple together electronic and electrical devices (e.g., integrated circuits, capacitors, resistors, etc.) mounted to the outer surface of the PCB. The insulating layers are positioned between signal layers to electrically isolate the signal traces on each signal layer. Plated vias or through holes are drilled through the PCB to facilitate electrical connections between the signal layers.
Many of the surface-mounted electronic devices and other components on a PCB commonly require coupling to individual resistors for termination, impedance matching, etc. The resistors are often mounted to mounting pads on the outer surface of the PCB. However, the mounting pads introduce capacitive and inductive impedance, which slows the speed of the circuit. Additionally, the resistors use valuable surface area that could otherwise be used for integrated circuits.
Accordingly, some PCB manufacturers use buried resistors, which are resistors located on inner signal or plane layers of the PCB. To create buried resistors, a signal layer is formed by electroplating a layer of nickel onto a layer of copper. This resulting bi-layered structure is glued to an insulating layer with the nickel layer adjacent to the insulating layer. Signal traces are etched so that each trace includes both copper and nickel layers. The exposed background nickel is removed by etching. Resistors are formed by further etching gaps in the copper layer of the race to expose the nickel below. Current flow along a signal trace normally flows through the less resistive copper layer, but at locations where the copper is etched away, current is forced to flow through the nickel. Since nickel is highly resistive relative to the copper, each gap in the copper layer forms a resistor having a value that is dependent on the width of the signal trace, the length of the gap in the copper, and the thickness of the nickel layer.
A prefabricated, bi-layered copper/nickel material is commercially available in large rolls from Ohmega Technologies, Inc. When manufacturing PCBs, the so-called Ohmega material or similar material must be unrolled and cut into sections the size of an insulating layer. The Ohmega material is then adhered to the insulating layer using glue or other adhesive.
Manufacturing PCBs using the Ohmega material or similar materials is costly and inefficient. The Ohmega material is difficult to work with because the large rolls of copper/nickel material are awkward to handle during manufacturing. Additionally, although the copper appears smooth to the naked eye, in actuality it is roughened to facilitate adhesion to the PCB when gluing. Such roughening causes peaks and valleys on the surface of the copper, and when the nickel is electroplated onto the copper, the nickel tends to be uneven, with a thicker deposition layer of nickel along the peaks and a thinner layer along the valleys. Because of the differing thicknesses of the nickel, the resistors formed from the nickel vary by as much as 20 percent. Such inconsistent resistor values are unacceptable and, consequently, many PCBs are scrapped as unusable. Additionally, gluing the Ohmega material to insulating layers is costly and reduces yield because of gluing imperfections. Still further, the use of Ohmega material limits the types of insulating materials that can be used as insulating layers because of incompatibility with the adhesives.
An objective of the invention, therefore, is to provide a method of forming a resistor on a PCB that has resistor values consistently within 5 percent of their desired value. A further objective is to provide a method of forming a resistor that has high yield and reduced cost.